1. Field of the Invention
The present invention relates to a patient ventilation system as well as to a gas identification method for use in such a patient ventilation system.
2. Description of the Prior Art
Patient ventilation systems are employed in the administration of breathing gas to a patient, particularly in a hospital environment, and operate to control either or both the amount and the composition of the administrated breathing gas. As such, in the present context, the term “ventilation system” shall encompass ventilators, respirators and anesthesia machines as well as on-demand type face masks employed in medical environments. An example of such a patient ventilation system is described in EP 1 455 876 B1.
Patients in need of frequent respiratory treatment often show a severe increase in airway resistance. To overcome that resistance, a certain gas pressure is needed for moving gas into and out of the lungs of the patient. The pressure in the airway is directly related to the dynamic pressure gradient during the respiratory cycle, the flow rate of the gas, the density and viscosity of the gas, and the caliber and length of the airway.
It is well known to mix air with oxygen to increase the overall oxygen concentration delivered to the patient. To decrease the pressure required for moving gas through the airways, air can be substituted by “heliox”, a mixture of helium and oxygen. As an inert gas, helium does not participate in any biochemical process of the body. However, as helium is the second lightest gas, it decreases the density and by that the required driving pressure. Typically, helium is mixed with at least 21% oxygen but depending on the specific conditions of the patient, this mixture can be altered.
Prior art ventilation systems normally have at least two gas inlets, one of which is connected to an oxygen source and the other to a second gas source such as an air source, a heliox source, a zenon source or a nitrous oxide source. If heliox is used, the distribution between helium and oxygen in the heliox mixture is typically 80% helium and 20% oxygen (heliox 80/20), or 70% helium and 30% oxygen (heliox 70/30). These external gas sources may be provided locally by pressurized bottles. Typically, there are often more gas supplies available for connection to the gas inlets than are required and care must be taken to ensure that the correct supplies are connected, especially as conventional gas sources are supplied with standardized pneumatic connection terminals. The prior art mentioned above discloses a gas identifier, which comprises a voltage divider adapted to provide an electrical interface to the ventilation system and a lookup table. The voltage divider includes a resistor having a resistance value unique for each gas supply. For a specific gas supply, a corresponding voltage drop will result as measured across the resistor. The lookup table comprises a list of voltage drops for the various gases, so that the gas mapping with the voltage drop is obtained from the lookup table.
With such an identification system, there may be an uncertainty if the correct voltage divider has been introduced or not. Therefore, the safety of such a system is deficient and barely provides more certainty than manually identifying the gas supply by simply looking at it and making the correct input to the ventilation system via the interface. In both cases and having in mind the stress situation in an ICU, there is no absolute knowledge about the gas, which actually is delivered to the ventilation system and there is no check up or safety control.
As is also known, e.g. from the prior art mentioned above, flow meters provide output signals which are dependent on the type of gas, i.e. if a flow meter is calibrated for measuring air, the meters output signal would deviate from the actual flow for another gas type like heliox 80/20. This is true even for other gases like nitrous oxide, zenon or other gas mixtures. The prior art therefore suggests means for correcting the calibration of any flow meter based on gas supply, which is identified in the above described way.
To ensure that a correct amount of oxygen is delivered to the patient, it is known to use an oxygen sensor, e.g. an oxygen cell, to measure the oxygen concentration in a gas mixture that is to be delivered to the patient. However, such an oxygen sensor cannot be used to identify what other gases or gas compositions, like air or heliox, are present in the gas mixture.
To increase the safety of any gas supply to a patient ventilation system, EP 1 441 222 A2 discloses monitoring means using an acoustic transceiver detecting the amplitude of the emitted acoustic energy propagated through a measurement chamber and generating a control signal from a comparison of the detected signal with a reference signal for the target gas, and generating a control signal to inhibit the gas flow through the system if the wrong gas is supplied.
It is also known in the art to measure the time of flight (TOF) for a sound pulse through a gas mixture, or the thermal conductivity of a gas mixture, in order to establish what other gases besides oxygen are present in that particular gas mixture.
However, since different gas compositions may have the same characteristics as regard the measured property (e.g. the ability of absorbing acoustic energy, conducting sound, conducting heat, etc.), the gas constituents of a gas mixture cannot always be unambiguously determined.